Notwithstanding the dramatic improvement in energy efficiency over more traditional light sources, light sources utilizing light emitting diodes (LEDs) still convert between 50 to 80% of the power they are fed into heat. At the same time, LED performance with respect to efficiency and color stability is quite sensitive to temperature increase, and especially for high temperatures above 80° C. This criticality is particularly evident in high power LED applications. Traditionally, heat sinks and forced air convection have been utilized for heat management of LED devices. More recently heat pipes have been employed for heat managing of LED devices. A heat pipe is an evaporator-condenser system in which a liquid is returned to the evaporator by capillary action. In its simplest form a heat pipe consists of a vacuum tight hollow tube with a wick structure along the inner wall, and a working fluid. The wick structure may be porous, such as sintered powder metal, wrapped, consist of axially arranged grooves, screens etc. The center core of the tube is left open to permit vapor flow. The heat pipe is evacuated and then back-filled with a small quantity of working fluid, just enough to saturate the wick. Examples of applicable working fluids are sodium, lithium, water, ammonia, and methanol. The atmosphere inside the heat pipe is set by an equilibrium of liquid and vapor. The heat pipe has three sections: evaporator, adiabatic and condenser. Heat applied at the evaporator section (also referred to as the hot part herein under) is absorbed by the vaporization of the working fluid. The vapor is at a slightly higher pressure, which causes it to travel down the center of the heat pipe, through the adiabatic section to the condenser section. At the condenser section (also referred to as the cold part herein under) the lower temperatures cause the vapor to condense giving up its latent heat of vaporization. The condensed fluid is then pumped back to the evaporator section by the capillary forces developed in the wick structure. Heat pipe operation is completely passive and continuous. This continuous cycle transfers large quantities of heat with very low thermal gradients. The operation of a heat pipe is passive, and is driven only by the heat that is transferred. In a gravity field, the evaporator may be placed below the condenser to assist the liquid flow. Heat pipes may be arranged in different shapes.
It is known to combine a heat sink, heat pipes and forced convection for heat management of LED based lighting devices. U.S. Pat. No. 7,144,135 B2, discloses a lighting device comprising a LED light source which is arranged on a heat sink. The heat sink is arranged with fins and/or heat pipes. An optical reflector encompasses the light source. The device further comprises an exterior shell in which the optical reflector is disposed such that an air channel is formed between the optical reflector and the shell. The fins and/or heat pipes of the heat sink are arranged to extend along the air channel. Further, a fan is arranged under the heat sink and causes air to flow from air inlets and air exhaust apertures defined by the shell/optical reflector such that the heat sink is cooled. In an exemplifying embodiment, a Luxeon 500 lm LED is cooled.